In January 2010, at the conclusion of the Australian Open, the tennis cognoscenti spoke as one. Roger Federer, who had just won his 16th major singles title, was the greatest tennis player in the sport’s history.

“He’s the best I’ve ever seen,” John McEnroe said. By this time Mac had seen Federer’s chief rival, Rafael Nadal, play hundreds of times.

Three-and-a-half years later, the fickle winds of tennis punditry have shifted. Now Nadal, with an incredible comeback season after losing seven months to knee problems, is increasingly viewed as the greatest of all time. Or at least the greatest-in-waiting. It seems clear that Federer’s career record for Grand Slam singles titles, now at 17, is destined to fall to his younger rival.

In this computer-dominated age, we are all slaves to numbers. And Nadal’s numbers speak loudly, especially his dominating head-to-head record against Federer.

But whoever becomes the consensus greatest of all time (and Nadal is indeed making a strong case for himself), let’s hope our imaginations can remain unimpeded by arithmetic. That’s what the greatest athletes have always done for us: they expand our dreams; they make the cold, hard reality of data seem slight, untenable.

Let’s consider Babe Ruth. The Sultan of Swat no longer holds Major League Baseball's record for home runs in either a season or a career. He’s fallen to second on the career runs-batted-in list and is 10th in batting average. He did not make it to 3,000 career hits, the standard for Hall of Fame hitters.

But so what? He remains, and will forever remain, the greatest baseball player who ever took a breath. That’s because Ruth single-handedly changed the nature of professional baseball, creating our ongoing love affair with the long ball and elevating the sport into the realm of cultural myth. He was, writes Bill Bryson in his new book One Summer: America, 1927, “the most brilliant, headstrong, undisciplined, lovable, thrillingly original, ornery son of a bitch that ever put on a baseball uniform.” Sixty years after Ruth passed into the ether, adolescent boys still know his name and, just as Ruth supposedly did, extravagantly point to the distant spot they plan to hit the ball.

This criterion for greatness, the criterion that lasts (because records, we all know, are made to be broken), is why John McEnroe retains a place in tennis' greatest-ever debate, despite his slight C.V. in comparison to those of Pete Sampras, Rod Laver, Federer and Nadal. Like Ruth with baseball, he was the player who, more than anyone else, remade tennis into a modern, big-time, blaring, bleating professional sport. It’s been argued that he was a much-needed real-life Holden Caulfield out to expose the old-school, country-club tennis establishment's phoniness. His gorgeous, “pointillist” tennis was a bonus that helped fuel the revolution.

This definition of greatness also bodes well for Federer, though for very different reasons. After the upheaval caused by McEnroe and his fellow rebels without causes in the 1970s and ’80s, Federer has come to represent the return of sophistication, of tennis as glorious, meritocratic elitism. The 32-year-old Swiss gives the sport an appealing, 21st-century imperial gloss just as it seeks to stretch out into previously unconquered territory in Asia and other rising locales. When Federer toured South America last year for a series of exhibitions, people poured into the streets to meet him at every stop. But it wasn't ordinary celebrity-worship at play; it was more like the British king in the early 1900s unexpectedly coming to tea at a commoner's flat. “They cry, and they shake, and they are just so, like, not in awe but so happy to meet you,” a stunned Federer told a reporter during the trip.

Such an image of Specialness, a category above and beyond mere celebrity, took hold in large part because of the kind of tennis Federer plays: extravagant, geometry-busting, god-like. This is the place for the obligatory "Federer Moments" quote from the late David Foster Wallace’s famous 2006 essay, which we’ll refrain from offering up since every tennis fan has memorized the piece by now. The essay’s existence and cultural ubiquity matter, though. Tennis scribes have been offering their own versions of it, in one way or another, for the past seven years.

Michael Steinberger, in his plea for us to remember the Swiss great despite his shocking early exit from this year’s U.S. Open, is the latest to call forth Wallace. Sports achievements are about winning, Steinberger acknowledged -- and Federer has done more than his fair share of that. But sport isn't just about winning. At its best, it’s also about adding a touch more beauty to the world.

“The fact that Federer, in addition to all the winning, has been able to conjure such ethereal tennis while matching the firepower of his rivals and at a time when so many things -- the rackets, the strings, the courts, the size, strength and speed of the players -- conspire against the expression of beauty in tennis, is testament to his greatness.”

In other words: Here we all are, still talking about “Federer moments.” And we will continue to talk about them in the years ahead since, unlike most of Laver’s great moments, they really will live on, thanks to the endless TV coverage that was unavailable during Rod the Rocket’s time.

Of course, it can’t be as simple as that. When Nadal went to South America early this year to work himself back into Grand Slam shape, his presence didn’t provoke anything close to the shocked, sobbing reaction that Federer’s did. This is chiefly about media-age image building, a triumph of marketing as much as personality. Federer has a regal quality about him that in many quarters overwhelms Nadal’s humble, earthy persona. But the thing is, we’ll be talking about Nadal moments, too, in the years ahead -- like the one below from the U.S. Open that quickly made its way around the world. His tennis isn’t “ethereal” like Federer’s; his game is more dependent on the modern rackets and strings and courts. But nevertheless, he also regularly makes our jaws drop. Beauty is in the eye of the beholder, and many thousands of serious fans observe great beauty in Nadal’s muscular tennis.

Ultimately, this is good for Federer. It is difficult to relate to or
even appreciate a great athlete once he becomes a legend. (Did Babe
Ruth really point to the right-field bleachers during a 1932 World
Series at-bat and then deposit the ball right where he had indicated?)
We tend to expect constant Federer Moments from Federer because, well,
he's Roger Federer. Rafael Nadal reminds us that his great rival is human,
which only makes us appreciate Federer's genius all the more.

Now
the question is becoming, Can the aging Swiss stay in Federer-like form
long enough to keep Rafa himself from being encrusted with the
expectations reserved for legends? With a little luck, he'll get one last chance in 2013 to help keep Rafa human -- by facing him at the ATP World Tour Finals, on the one surface (indoor hard court) where he remains unquestionably superior. And so the debate will rage on.